1. Field of the Invention
This invention relates to optical devices and, more particularly, to electrochromic devices.
2. Art Background
A variety of electrochromic devices have been proposed recently. Among these devices are those which utilize an electrochromic iridium oxide electrode together with a counterelectrode such as a platinum or an iridium oxide counterelectrode. In this iridium oxide device, the iridium oxide electrochromic electrode is grown, for example, by a vacuum deposition process such as sputtering or by a cyclic anodization process. The device is constructed by contacting the electrochromic electrode and the counterelectrode with a suitable electrolyte such as an aqueous sulfuric acid solution.
Electrochromic devices generally exhibit open circuit memory, i.e., the colored state of the electrochromic electrode persists even though the circuit between the electrochromic electrode and the counterelectrode is opened and no voltage is applied. (See, for example, A. R. Kmetz and S. K. Von Willisen, Non Emissive Electro Optic Displays, Plenum, New York, 1976.) Although open circuit memory is an important property, matrix addressing schemes used in display configurations also require a threshold coloration potential and a short circuit memory. (These properties should co-exist with a suitable response time, e.g., for many applications less than one second for coloration and bleaching.)
The presence of a threshold potential is determined by comparison to the two extreme voltages utilized in the operation of the device. The upper extreme voltage is that necessary to produce a desired optical density change upon coloration, the lower extreme typically is the voltage at which degradation, e.g., hydrolysis of an aqueous electrolyte, occurs. For threshold, the voltage at which charge exchange with the electrolyte begins should be larger than the lower extreme by one-half or more of the difference between the upper and lower extremes of the operating voltages. In many matrix addressing schemes a cell within a multi-element display device is colored by applying a voltage to the appropriate row and column of an array. The voltage applied in either the row or column is insufficient to color the device but the two voltages together induce coloration. To insure this additive effect is posible, the cell should have a coloration voltage threshold.
The second necessary property, short circuit memory, is the persistence of color after reduction of the applied potential below the coloration threshold while electrical continuity between the two electrodes is maintained. Because of the electronic configuration employed in matrix addressing matrix elements having battery characteristics, e.g., electrochromic cells are short circuited during operations involving the coloration of other cells within the matrix. If the elements do not have short circuit memory, parts of the displayed information in the array are lost during the operation necessary to display information at other portions of the array.
Since matrix addressing is a desirable approach to producing display systems, the development of an electrochromic device having such short circuit memory and threshold would be quite significant. However, an electrochromic device, in particular an iridium oxide based device with both short circuit memory and threshold, has not been disclosed.